Photosynthesis is a very important metabolic pathway for plant growth and crop yield. This report investigated the effect of the herbicide imazethapyr on photosynthesis in the Arabidopsis thaliana pnsB3 mutant (a defect in the NDH pathway) and pgr5 mutant (a defect in the PGR5 pathway) to determine which cyclic electron transport chain (CET) of the NDH and PGR5 pathways is more important for protecting the photosynthetic system under herbicide stress. The results showed that 20 μg/L imazethapyr markedly inhibited the growth of the three ecotypes of A. thaliana and produced more anthocyanins and reactive oxygen species (ROS), particularly in the pgr5 mutant. The chlorophyll fluorescence results showed that PSII was severely damaged in the pgr5 mutant. Additionally, the CET was significantly stimulated to protect the photosynthetic system from light damage in Wt and the pnsB3 mutant but not the pgr5 mutant. The real-time PCR analysis indicated that imazethapyr treatment considerably decreased the transcript levels of most photosynthesis-related genes in the three treated groups. Several genes in the PGR5 pathway were significantly induced in the pnsB3 mutant, but no genes in the NDH pathway were induced in the pgr5 mutant. The gene transcription analysis showed that the pgr5 mutant cannot compensate for the deficit in the PGR5 pathway by stimulating the NDH pathway, whereas the pnsB3 mutant can compensate for the deficit in the CET cycle by regulating the PGR5 pathway. The iTRAQ analyses also showed that the photosynthesis system, glycolysis, and TCA cycle suffered the most severe damage in the pgr5 mutant. All of these results showed that the PGR5 pathway is more critical for electron transfer around PSI than the NDH pathway to resist herbicide stress.
Keywords: Arabidopsis thaliana; iTRAQ; imazethapyr; mutant; photosynthesis.